A method for converting analog data into digital data for analog input module is disclosed, wherein the analog input module adds at least one or more digital data determined at previous conversion periods (k−1th, . . . k−dth, said k and d being a natural number) to n numbers of digital data converted to the current conversion period (kth), and determines a value corresponding to a mean level in size among the digital data as a final digital conversion value of the current conversion period (kth), and outputs the value, and as a result, even if the n numbers of digital data converted to the current conversion period (kth) are greatly deviated from an original value by noise, a stable result having a minimum influence by the noise can be provided in industrial sites where many noises are introduced, by allowing a digital data value approximate to the original value to be outputted.